Proton therapy has become a significant radiation therapy around the world with more than ten facilities currently operating in the United States alone and several more in the making. Furthermore, although other light ions, e.g., deuterons, tritons, He-3 and He-4 ions, Li-6 and Li-7 ions, and ions of beryllium and boron, have not been used for radiation therapy, several types of accelerators currently used for proton and carbon therapy are capable of accelerating such light ions. Finally, radiation therapy with carbon ions, although not in clinical use in the United States yet, has been in clinical use in Japan and Germany for close to 15 years.
As used herein, the term “charged particles” refers generally to ions of elements from the periodic table of elements, of any atomic number.
In addition, the term “particle therapy,” as used herein, refers to radiation therapy using any charged particle or ion.
While the term “light ions” may be used to refer to any ions or charged particles from protons to neon (from Z=1 to 10 inclusive), the methods of the present disclosure are particularly suited for ions and charged particles from protons to boron.
The advantages of light ions for radiation therapy over x-rays and gamma rays used in conventional radiation therapy are mostly their Bragg peak feature of dose deposition in tissues that allows better confinement of the dose to the target, as depicted in FIG. 2. FIG. 1 compares the dose deposition 10 with depth in tissues 12 of protons and carbon ion beams with that of high energy x-rays called MV x-rays because they are produced by MV electrons linacs. Proton therapy and carbon therapy of tumors are implemented by spreading the Bragg peak 14 to produce a flat dose 16 over the length of the tumor with only little exit dose (FIG. 2). Representative plots showing the Bragg peak for different energies 14 of carbon ions are shown as an example in FIG. 2, along with the resultant flat dose 16 resulting from Bragg-peak spreading across the energies. However, protons, carbons, and other charged heavy particles have a major disadvantage over MV x-rays and gamma rays used in Gamma Knife and that is that they do not have the sparing effect of the shallow tissues of the MV x-rays and gamma rays, an effect loosely called “skin-sparing effect”, which is demonstrated in FIG. 1 as a large dip in the entrance dose of MV x-rays. Therefore, despite the fact that the Bragg-peak-spread dose protons and carbons deliver to the target is larger than their entrance doses, their entrance dose is still much higher than that of MV x-rays and gamma rays. This limitation of protons, carbon ions, and particle beams in general, combined with the very high radiosensitivity of the skin and certain other shallow tissues, such as the brain's frontal cortex, as described below, limits the entrance dose in each therapy session from these particles as compared to possible entrance doses from MV x-rays and gamma rays. Therefore, in contrast to MV x-rays and gamma rays that, depending on the size of the target, can be given in a single session (called single dose fraction) or just a few sessions, proton therapy is mostly administered in 20 to 30 dose fractions, which would be four to six weeks of five treatments each week. Although the number of dose fractions used in carbon therapy is generally smaller because of the high relative biological effectiveness (RBE) of carbon ions, which is mostly at their Bragg Peak, it still ranges from 5 to 15 because of their above lack of sparing effect of shallow tissues.
The sparing effect in shallow tissues by MV x-rays and gamma rays is well known, and is a result of the mechanism by which the dose is deposited. Being neutral particles, the x-rays and gamma rays deposit a dose by setting electrons in motion via either the photoelectric effect, Compton scattering, or pair-production, which in turn deposit the dose in tissue. For MV x-rays, Compton scattering is the dominant mode of interaction with tissues. The population of the electrons set in motion by the incident x-rays is built up only gradually, and typically it takes a centimeter or so for the built-up electron density to reach the equilibrium state. The depth and the shape of the “tissue sparing curve” of MV x-rays depend on the energy of the MV x-rays or gamma rays. It ranges from several millimeters for 6 MV linacs to about 15 mm for the 18 MV ones (FIG. 1). In that regard the curve in FIG. 1, made for 21 MeV x-rays, is highly atypical and is therefore exaggerated for our discussion.
On the other hand, protons, being charged particles, start depositing their energy into tissue immediately as they enter it. Accordingly, there is no such shallow tissue-sparing effect as there is for MV x-rays and gamma rays. This limits the entrance dose from these particles in comparison to possible entrance doses of MV x-rays and gamma rays.
Accordingly, there is a need for an effective radiation therapy using protons or other ions, particularly light ions, which advantageously confines the radiation dosage to the target, and that also offers shallow tissue and skin-sparing.